Implants are utilized in modern medical technology in a variety of embodiments. They are used e.g. to support vessels, hollow organs, and ductal systems (endovascular implants e.g. stents), to attach and temporarily fix tissue implants and tissue transplants in position, and for orthopedic purposes such as pins, plates, or screws. The stent is a form of an implant that is used particularly frequently.
Stent implantation has become established as one of the most effective therapeutic measures for treating vascular disease. Stents are used to provide support in a patient's hollow organs. For this purpose, stents of a conventional design have a filigree support structure composed of metallic struts; the support structure is initially provided in a compressed form for insertion into the body, and is expanded at the application site. One of the main applications of stents of this type is to permanently or temporarily widen and hold open vasoconstrictions, in particular constrictions (stenoses) of the coronary arteries. In addition, aneurysm stents are known, for example, which are used to support damaged vascular walls.
Stents include a circumferential wall having a support force that suffices to hold the constricted vessel open to the desired extent; stents also include a tubular base body through which blood continues to flow without restriction. The circumferential wall is typically formed by a latticed support structure that enables the stent to be inserted, in a compressed state having a small outer diameter, until it reaches the constriction in the particular vessel to be treated, and to be expanded there, e.g. using a balloon catheter, until the vessel finally has the desired, enlarged inner diameter.
The implant, in particular the stent, has a base body composed of an implant material. An implant material is a nonliving material that is used for a medical application and interacts with biological systems. A prerequisite for the use of a material as an implant material that comes in contact with the physical surroundings when used as intended is its biocompatibility. “Biocompatibility” refers to the capability of a material to evoke an appropriate tissue response in a specific application. This includes an adaptation of the chemical, physical, biological, and morphological surface properties of an implant to the recipient tissue, with the objective of achieving a clinically desired interaction. The biocompatibility of the implant material is furthermore dependent on the time sequence of the response of the biosystem in which the implant is placed. For example, irritations and inflammations, which can cause tissue changes, occur over the relative short term. Biological systems therefore respond differently depending on the properties of the implant material. Depending on the response of the biosystem, implant materials can be subdivided into bioactive, bioinert, and degradable/resorbable materials.
Implant materials include polymers, metallic materials, and ceramic materials (as a coating, for example). Biocompatible metals and metal alloys for permanent implants contain e.g. stainless steels (e.g. 316L), cobalt-based alloys (e.g. CoCrMo casting alloys, CoCrMo forging alloys, CoCrWNi forging alloys, and CoCrNiMo forging alloys), pure titanium and titanium alloys (e.g. CP titanium, TiAl6V4 or TiAl6Nb7), and gold alloys. In the field of biocorrodible stents, the use of magnesium or pure iron and biocorrodible base alloys of the elements magnesium, iron, zinc, molybdenum, and tungsten is proposed. The present invention relates to biocorrodible base alloys, in particular base alloys of magnesium.
To perform radiological intraoperative and postoperative position monitoring, implants are provided with at least one marker if they are not already composed of a sufficiently radio-opaque material. The X-ray visibility of the marker is a function of the dimensions and X-ray absorption coefficient thereof. The X-ray absorption coefficient is, in turn, a function of the energy range of the X-ray radiation: it is typically 60 to 120 keV in the medical field, and 60 to 100 keV for coronary applications. The X-ray absorption coefficient typically increases as the atomic number in the periodic table rises and the density of the material increases. The presence of the marker should not restrict the functionality of the implant and/or be the starting point for inflammatory responses or rejection reactions of the body. Typically, for example, noble metals, such as gold and platinum, are used as marker materials.
The markers are provided (i) as solid material e.g. in the form of a coating, a strip, an inlay, or a molded body permanently bonded to the implant, or (ii) a powder embedded in a carrier matrix, in the form of a coating or as a filler material for a cavity in the implant. Variant (ii) can be implemented particularly easily in terms of production technology: A castable or sprayable mixture of the radio-opaque marker components and the material acting as a carrier matrix, possibly with a solvent added, is processed.
The biocorrodible metal alloys known from the prior art for use in medical implants have only slight X-ray visibility in the energy range of 60-100 keV, which is used for medical technology. However, X-ray diagnosis is an important instrument precisely for postoperative monitoring of the healing process or for checking minimally invasive interventions. Thus, for instance, stents have been placed in the coronary arteries during treatment of acute myocardial infarction for some years. The stent is positioned in the area of the lesion of the coronary vascular wall and prevents obstruction of the vascular wall after expansion. The procedure of positioning and expanding the stent must be continuously monitored by the cardiologist during the procedure.
For implants composed of biocorrodible metallic materials based on magnesium, iron, or tungsten, there are increased requirements on the marker material, which include:                the marker is not to be detached prematurely from the base body of the implant by the corrosive processes, to avoid fragmentation and thus the danger of embolization;        the marker is to have sufficient X-ray density even when material thicknesses are low, and        the marker material is to have no or, at most, a slight influence on the degradation of the base body.        
However, when markers are used that are composed of metallic materials on biocorrodible metallic base bodies, the particular problem arises that, due to electrochemical interactions between the two metallic materials, the degradation of the base body is altered in a contact region between the marker and the base body, i.e. the degradation is typically accelerated. DE 10 2008 043 642 A1 describes an endoprosthesis comprising a voluminous marker provided with a barrier layer, using which the radio-opaque material of the marker is electrically insulated from the base body. The base body can be composed of a biocorrodible magnesium alloy.